Vibrational spectroscopy is emerging as an important tool in the structural characterization of macromolecular ions generated using electrospray ionization (ESI). This is evidenced by the explosion of papers reporting isomer analysis by comparison of vibrational action spectra obtained by infrared multiphoton dissociation (IRMPD) with predictions from electronic structure calculations.[1-6] There are, however, complications in this strategy because the intrinsic non-linearity of the IRMPD method obscures direct comparison with the harmonic absorption spectrum that is readily computed using commercial software packages.[7] Moreover, the fluxional nature of the molecules gives rise to many isomers at low temperature which can often interconvert under ambient conditions. [8] A powerful way to overcome these limitations is through the use of so-called “messenger spectroscopy,”[9, 10] where the ion of interest is complexed with a weakly bound ligand (such as a rare gas atom), and the vibrational action spectrum is monitored by photoinduced loss of the messenger. In this approach, the ion is intrinsically cooled to an upper limit defined by the messenger binding energy, and efficient intramolecular vibrational energy redistribution upon excitation in the fingerprint region of the infrared leads to prompt ejection of the messenger. The resulting action spectra are linear in laser intensity, with a few notable exceptions,[11] and therefore more accurately reflect the linear absorption profiles associated with specific local minimum structures of the target molecule or cluster. This method has been widely used to study ions and ion-solvent clusters that can be prepared using supersonic jet technology.[10, 12-29] On the other hand, application of this method to the classes of ions that can only be generated with ESI is still in its infancy, with a notable recent paper reporting spectra of Kr tagged suberate dianions using a temperature controlled ion trap subsequent to ion generation. [30]
Pursuant to the present invention, the inventors present vibrational predissociation spectra of the anions generated from sequential deprotonation of dodecanedioic acid using H2 as a messenger, where the recent demonstration by Wang and co-workers[31, 32] that large numbers of H2 molecules (up to 12) can be efficiently attached to multiply charged ions from an ESI source using a 10 K radio-frequency (RF) quadrupole ion trap. This is significant because H2 is often non-reactive and quite weakly bound to a variety of closed-shell molecular ions prepared by ESI, and was, in fact, one of the species used in the 1980s for the first reports of the messenger technique.[10, 21, 22] In the case of H5O2+, for example, the binding energy and perturbation induced by H2 were on the same order as that found for Ar tagging.[21, 33] In the present invention, the inventors extend the trap-based methods to singly charged anions by pulsing the H2/He mixture into the trap, and report the resulting vibrational spectra of the HOOC(CH2)10COO− and −OOC(CH2)10COO− ions over the range 800-4300 cm−1. These data are interpreted within the context of a closed, H-bonded ring form for the singly charged species, an arrangement that was inferred by Woo et al.[34] from their analysis of the photoelectron spectra of this species.